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Help: Identify this Diode (schematic)

Discussion in 'Electronic Components' started by Jerry Lynds, Apr 17, 2007.

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  1. Jerry Lynds

    Jerry Lynds Guest

  2. Oppie

    Oppie Guest

    The outputs that feed the Zeners are AC coupled with a zener and a 100K to
    ground. My guess is that the zeners just clamp the voltage to a safe level
    for the 6739 inputs. Look at the 6739 data sheet and find out if there are
    any input voltage constraints for starters.

    A bit suprising that Zeners would be used to clamp video lines...since they
    typically have a high capacitance. This leads me to think that they may be
    some other type of clamp diode.
    Oppie
     
  3. I don't think these are zeners, but Schottkys.

    The capacitive coupling is followed by 10k resistors to
    ground, that would otherwise try to center the signal swing
    at zero volts. But the diodes clamp the negative swing to
    about -.3 volts, by charging the capacitors positively, so
    that, as long as the signal cycle occurs much quicker than
    the RC time constant, instead of being centered on ground,
    it has a maximum negative excursion just below ground. So
    the diodes are part of a level shifting scheme. I suspect
    almost any small signal Schottky diode would work, as long
    is it has a voltage rating well above 5 volts.
     
  4. Eeyore

    Eeyore Guest

    I agree 100%.

    Graham
     
  5. Ray King

    Ray King Guest

    Jerry,
    I think you are correct. these are schottkys. these 100uf caps can put quite
    a negative punch with out limiting the charge current. I would use no less
    than the IN5817 no 18 nor 19.
    Ray
     
  6. Jerry Lynds

    Jerry Lynds Guest

    You guys are awesome! Thanks a million. You saved me a lot of stress. I'll
    pick some up tomorrow

    Cheers,

    Jerry
     
  7. The capacitors are not the limiting impedance for these
    signals. The outputs are not low impedance sources. That
    is why buffer amps are needed.

    Here is a clip from the data sheet:

    "The LMH1251 is designed to interface with an ADC or
    preamplifier through an AC coupling capacitor as shown
    below in Figure 1. The RGB outputs of the LMH1251 are 700
    mVPP video signals with the black level at approximately 2V,
    which is the chip’s internal voltage reference level."

    "The RGBHV outputs cannot be used drive standard 150-
    ohm video loads and require high-bandwidth buffers for this
    kind of application. For example, if the LMH1251 is to be
    designed into a stand-alone converter box application, the
    configuration in Figure 2 is recommended. To drive a display
    monitor over a standard VGA cable, a wideband, low distortion
    triple video buffer, such as the LMH6739, can be used
    with a gain of +2 to drive the RGB video signals of the
    LMH1251, and logic inverters can be used to drive its H and
    V sync signals."

    I don't see that spec on the output impedance, but it must
    be quite a bit higher than 150 ohms.

    So any small signal Schottky should be big enough.
     
  8. Ray King

    Ray King Guest

    John,
    If you know the output pull down current of the driver then what ever that
    current is should be applied to the schottky and this should not exceed 0.55
    volts drop in the schottkys forward direction. Is my guess.
    Ray


    The capacitors are not the limiting impedance for these
    signals. The outputs are not low impedance sources. That
    is why buffer amps are needed.

    Here is a clip from the data sheet:

    "The LMH1251 is designed to interface with an ADC or
    preamplifier through an AC coupling capacitor as shown
    below in Figure 1. The RGB outputs of the LMH1251 are 700
    mVPP video signals with the black level at approximately 2V,
    which is the chip’s internal voltage reference level."

    "The RGBHV outputs cannot be used drive standard 150-
    ohm video loads and require high-bandwidth buffers for this
    kind of application. For example, if the LMH1251 is to be
    designed into a stand-alone converter box application, the
    configuration in Figure 2 is recommended. To drive a display
    monitor over a standard VGA cable, a wideband, low distortion
    triple video buffer, such as the LMH6739, can be used
    with a gain of +2 to drive the RGB video signals of the
    LMH1251, and logic inverters can be used to drive its H and
    V sync signals."

    I don't see that spec on the output impedance, but it must
    be quite a bit higher than 150 ohms.

    So any small signal Schottky should be big enough.
     
  9. My point was, that if the output impedance is more than 150
    ohms, and the signal swing is no more than .7 volts, then
    the peak output current can't be more than a few
    milliamperes, so 1 amp rated Schottky diodes are not needed.
    And the capacitance of such a big diode will have a bad
    effect on the frequency response of the signal.
     
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